I bet you the noise you're making is an "A". In fact, if it isn't, you're only making things difficult for me and for yourself, because A and its role in crosswords make up the topic for today.

Aye, it's a' about the A. It may be only the third most common letter in English - and by implication about the same in crosswords - but it is, without doubt, the best. You need only think of ace, or Alan, or A-number-1 to see that A denotes awesomeness - the letter against which all others are judged and found wanting.

A is a useful letter for setters and solvers alike, as befits its putative origins as the representation of the head of an animal valued both for pulling things and for being tasty - the ox. (You might right at this moment be among those contesting that story, quite reasonably objecting that if you had an ox with a head shaped like an A, you'd take the poor cone-head straight to the vet - but the story goes that the Greeks turned upside-down a Phoenician letter that was originally written ∀, sometimes with curly horns and/or some cute facial features.)

Much of this usefulness comes from the way in which A is a word as well as a letter. So often an "A" in an answer will be indicated by "one". It's a smart feature of English: to have such a short, easy way of saying "a". Pity the Germans, the Dutch and the Greeks with their eines, their eens and their εvαs or whatever they use. A might be just an "an" - that is to say, a "one" - with the "n" left out because you're about to say a consonant, but it's short and it's nifty and I like it a lot. (It's worth knowing a few of those foreign "a"s, mind, for when a setter uses "A French" at the start of a clue to indicate UN- at the start of an answer.)

"One" can of course also indicate the numeral's lookalike, the capital I. But when "one" or "a" itself is used to indicate an A in an answer, it helps the setter construct a clue with a natural surface reading. There are, to be sure, less obvious ways of hinting at A.

Adult, for example. For the benefit of younger readers, some films used to receive a tantalising A certificate, shorthand among prurient teenagers for "not a guarantee of nudity and violence, but the odds are definitely better". The rating may now be (1) called "PG" and (2) effectively meaningless but the abbreviation lives on in crosswords, along with the abolishedacre and still more antiquated uses, like before, year and in the year, via the Latin ante, annus and anno, all of which you'll find under A in a puzzle-friendly dictionary. (That, I suppose, is the main criterion for whether a word or series of words indicates an A, though moderately advanced solvers seem to just pick them up and get used to them as opposed to rooting around in Chambers.)

More usual than that Latin trio - that is, what you expect to find in a daily broadsheet cryptic - are America (as in "USA") and American, Australia and Australian - and Austria. Not "Austrian", though - in the case of Austria, it's from the license-plate code. Also common are amateur, area, absent and accepted.

More deranged than that Latin trio - what you should find only in sadistic weekend puzzles - are words like are. An are is - yes, you read that right - an are is the unit of metric land measure, 100 square metres, abbreviated to "a" - though of course that's not how any solver is going to initially read "are" in a clue. Only those who pit themselves against the pitiless setters of the Listener, Enigmatic Variations and so on need worry about are, or that A appears to be an all-purpose pronoun in some UK dialects - so "locally, they" or "she, in parts" could mean an A. And let's not even go near the Ångström or the medieval Roman numeral for 5,000.

More sensible to note that A can also be denoted by note (it's the one that follows G), and per or for each (as in the one about the man taking two dead badgers on to a plane and being told "only one carrion per passenger, Sir") and to remember that if you're reading aloud the checking letters in a final unsolved clue in the hope that the word will magically appear on your lips, it's worth saying any "A"s a few different ways. Like all the ways in the sentence "her bare-faced father was a watering can".

Finally an absolute word of reassurance to the crossword amateur: A is most often indicated in a cryptic clue by, well, "a". It's a device that's sometimes easy to overlook, but you can hardly say it's not fair. So let's hear it for A. An honest, hard-working character. As the Canadians would put it: what a letter, eh?